This invention relates to a modulation device used for modulating with video and audio signals and, more particularly, to a modulation device suitable for use at the head end of a joint reception device.
As disclosed, for example, by Hitoshi Honda et al in "Future Promotion of the Cable TV System Applications", in pages 542 to 561 of The Official Transcript for 20th Annual NCTA Convention, July 6-9, 1971, at the head end of a joint TV reception device, it is a general practice to modulate a carrier wave with baseband video and audio signals obtained by demodulating a received satellite broadcasting signal which has been actively developed recently to obtain a TV broadcasting signal which is transmitted further. To this end, a plurality of modulation devices as shown in FIG. 3 (based upon FIG. 5 of the reference) are installed and provided respectively with baseband video and audio signals indicative of mutually different information contents, thereby forwarding respective outputs as TV broadcasting signals of different channels. Each of these modulation devices includes a modulator 2, a first mixer 4, a fixed oscillator 6, a band-pass filter (BPF) 8, a second mixer 10, a variable oscillator 12, a low-pass filter (LPF) 14 and a wide band, high power amplifier 16; and only the oscillation frequency of variable oscillator 12 is made different from those of the other modulation devices.
More particularly, the modulator 2 modulates a carrier wave of 45.75 MHz for example with the video signal and another carrier wave of 41.25 MHZ for example with the audio signal and supplies the resultant modulated signal f.sub.0 to the first mixer 4. The first mixer 4 mixes the modulated signal f.sub.0 with an oscillation signal f.sub.1 (e.g., 567 MHz) from the fixed oscillator 6 and supplies mixed output signals f.sub.0 +f.sub.1 and f.sub.0 -f.sub.1 to the band-pass filter 8. The band-pass filter 8 extracts one of these mixed signals, for example, f.sub.0 +f.sub.1 and supplied it to the second mixer 10. The second mixer 10 mixes the mixed signal f.sub.0 +f.sub.1 with an oscillation signal f.sub.i (i=1, 2, . . . n) and supplies the mixed output signals F.sub.i +(f.sub.0 +f.sub.1) and F.sub.i -(f.sub.0 +f.sub.i) to the low-pass filter 14. The frequency F.sub.i is previously selected so that one of these mixed output signals have a frequency needed in the corresponding TV channel of each modulation device. The low-pass filter 14 passes the necessary frequency signal, F.sub.i -(f.sub.0 +f.sub.1) for example, and supplies it to the wide band, high power amplifier 16. The amplifier 16 amplifies the signal F.sub.i -(f.sub.0 + f.sub.1) for further transmission.
Such modulation device is advantageous in that it is possible to obtain a TV broadcasting signal of any channel by merely adjusting the oscillation frequency of the variable oscillator 12. For this purpose, the cutoff frequency of the low-pass filter 14 is selected, for example, as 300 MHz so that the filter 14 can pass the TV broadcasting signal which is highest in frequency of those provided by respective modulation devices. The wide band, high power amplifier 16 is also arranged to have a rather wide band width, for example, as 50 MHz to 300 MHz. so that it can amplify all TV broadcasting signals provided by the first to n-th modulation devices of the lowest to highest frequencies.
However, the output signal of the wide band, high power amplifier 16 may include a higher harmonic component and, if such component is delivered as it is, it may interfere the other equipments in which the higher harmonic frequency is used. For example, when the output frequency band of a specific modulation device is the channel 2 of U.S.A. where the carrier frequency of modulated video signal is 55.25 MHz, the carrier frequency of modulated audio signal is 59.75 MHz and their second higher harmonic frequencies are therefore 110.5 MHZ and 119.5 MHz, respectively, these harmonic frequencies will interfere the channel A-2 and A-1 within the 84-174 MHz band allotted to the U.S. CATV line. Although the frequency f.sub.1 of the fixed oscillator 6 of the above-mentioned modulation device is selected as high as 567MHz, if it is low, unnecessary one of the two mixed outputs of the second mixer 10 may pass the low-pass filter 14 to be amplified by the amplifier 16 for delivery and the danger of interference is still maintained.